The object of the research proposed here is to gain a better understanding of both the genetic and molecular basis of the interaction of enteric microorganisms with epithelial cells. We have focused on three aspects of this interaction: entry of the bacteria (Yersinia and Salmonella) into epithelial cells; replication of the bacteria within the epithelial cell; and transcytosis of the bacteria through the epithelial cell. We will continue to characterize the invasion determinants, inv and ail, of Yersinia both genetically and biochemically. In vitro modified inv and ail genes will be returned to Yersinia to better assess the role these determinants play in the virulence of this organism. In addition, we will extend our studies with Salmonella to identify and characterize the genes (and their products) involved in invasion and intracellular replication. Our preliminary work in these areas has been most encouraging and indicates these aspects of microbial pathogenesis are amenable to experimental investigation.